Peptides specific for Mycobacterium avium subspecies paratuberculosis infection: diagnostic potential

Mycobacterium avium subspecies paratuberculosis (Map) is the causative agent of Johne's disease (JD). Current serological diagnostic tests for JD are limited by their sensitivity when used in sub-clinical stages of the disease. Our objective was to identify peptides that mimic diagnostically important Map epitopes that might be incorporated into a new-generation JD diagnostic. Four peptides were isolated from a phage-displayed random peptide library by screening on antibodies derived from Map-infected goats. The peptides were recognised by antibodies from Map-infected goats but not by antibodies from uninfected goats. The peptides elicited immune responses in rabbits, which reacted strongly with bona fide Map antigens proving the peptides were true epitope mimics. To assess the diagnostic value a panel of goat sera was screened for reactivity's with peptides. The peptides were recognised by antibodies from a proportion of goats infected with Map compared with control animals with a diagnostic specificity of 100% and the sensitivity ranged from 50 to 75%. Combinations of any two peptides improved sensitivity 62.5-87.5% and 100% sensitivity was achieved with three of the four peptides in combination. These data suggest peptides representing diagnostically important Map epitopes could be incorporated into a sensitive diagnostic test.

Peptide mimics selected from immune sera using phage display technology can replace native antigens in the diagnosis of Epstein-Barr virus infection

There is an expanding area of small molecule discovery, especially in the area of peptide mimetics. Peptide sequences can be used to substitute for the entire native antigen for use in diagnostic assays. Our approach is to select peptides that mimic epitopes of the natural immune response to Epstein–Barr virus (EBV) that may be recognised by antibodies typically produced after infection with EBV. We screened a random peptide library on sera from rabbits immunised with a crude preparation of EBV and serum antibodies from a patient with a high titer of EBV antibodies. We selected four peptides (Eb1–4) with the highest relative binding affinity with immune rabbit sera and a single peptide with high affinity to human serum antibodies. The peptides were coupled to the carrier molecule BSA and the recognition of the peptides by IgM antibodies in clinical samples after infection with EBV was measured. The sensitivities were Eb1 94%, Eb2, 3, 4 88%, H1 81% and all had 100% specificity. This study illustrates that the phage display approach to select epitope mimics can be applied to polyclonal antibodies and peptides that represent several diagnostically important epitopes can be selected simultaneously. This panel of EBV peptides representing a wide coverage of immunodominant epitopes could replace crude antigen preparations currently used for capture in commercial diagnostic tests for EBV.

RNA editing in hepatitis delta virus

Hepatitis delta virus (HDV) relies heavily on host functions and on structural features of the viral RNA. A good example of this reliance is found in the process known as HDV RNA editing, which requires particular structural features in the HDV antigenome, and a host RNA editing enzyme, ADAR1. During replication, the adenosine at the amber/W site in the HDV antigenome is edited to inosine. As a result, the amber stop codon in the hepatitis delta antigen (HDAg) open reading frame is changed to a tryptophan codon and the reading frame is extended by 19 or 20 codons. Because these extra amino acids alter the functional properties of HDAg, this change serves a critical purpose in the HDV replication cycle. Analysis of the RNA secondary structures and regulation of editing in HDV genotypes I and III has indicated that although editing is essential for both genotypes, there are substantial differences. This review covers the mechanisms of RNA editing in the HDV replication cycle and the regulatory mechanisms by which HDV controls editing.

The Woodchuck Model of HDV Infection

The Eastern woodchuck, Marmota monax, has been a useful model system for the study of the natural history of hepadnavirus infection and for the development and preclinical testing of antiviral therapies. The model has also been used for hepatitis delta virus (HDV). In this chapter several new applications of the woodchuck model of HDV infection are presented and discussed. The development of a woodchuck HDV inoculum derived from a molecular clone has facilitated the analysis of viral genetic changes occurring during acute and chronic infection. This analysis has provided insights into one of the more important aspects of the natural history of HDV infection-whether a superinfection becomes chronic. These results could renew interest in further vaccine development. An effective therapy for chronic HDV infection remains an important clinical goal for this agent, particularly because of the severity of the disease and the inability of current hepadnaviral therapies to ameliorate it. The recent application of the woodchuck model of chronic HDV infection to therapeutic development has yielded promising results which indicate that targeting the hepadnavirus surface protein may be a successful therapeutic strategy for HDV.

The most polymorphic residue on Plasmodium falciparum apical membrane antigen 1 determines binding of an invasion-inhibitory antibody

Apical membrane antigen 1 (AMA1) is currently one of the leading malarial vaccine candidates. Anti-AMA1 antibodies can inhibit the invasion of erythrocytes by Plasmodium merozoites and prevent the multiplication of blood-stage parasites. Here we describe an anti-AMA1 monoclonal antibody (MAb 1F9) that inhibits the invasion of Plasmodium falciparum parasites in vitro. We show that both reactivity of MAb 1F9 with AMA1 and MAb 1F9-mediated invasion inhibition were strain specific. Site-directed mutagenesis of a fragment of AMA1 displayed on M13 bacteriophage identified a single polymorphic residue in domain I of AMA1 that is critical for MAb 1F9 binding. The identities of all other polymorphic residues investigated in this domain had little effect on the binding of the antibody. Examination of the P. falciparum AMA1 crystal structure localized this residue to a surface-exposed alpha-helix at the apex of the polypeptide. This description of a polymorphic inhibitory epitope on AMA1 adds supporting evidence to the hypothesis that immune pressure is responsible for the polymorphisms seen in this molecule.

Tumour targeting of humanised cross-linked divalent-Fab' antibody fragments: a clinical phase I/II study

Antibody engineering has made it possible to design antibodies with optimal characteristics for delivery of radionuclides for tumour imaging and therapy. A humanised divalent-Fab' cross-linked with a bis-maleimide linker referred to as humanised divalent-Fab' maleimide was produced as a result of this design process. It is a humanised divalent antibody with no Fc, which can be produced in bacteria and has enhanced stability compared with F(ab')(2). Here we describe a clinical study in patients with colorectal cancer using humanised divalent-Fab' maleimide generated from the anti-carcinoembryonic antigen antibody A5B7 radiolabelled with iodine-131. Ten patients received an i.v. injection of iodine-131-labelled A5B7 humanised divalent-Fab' maleimide, and positive tumour images were obtained by gamma camera imaging in eight patients with known lesions, and one previously undetected lesion was identified. True negative results were obtained in two patients without tumour. Area under the curve analysis of serial blood gamma counting and gamma camera images showed a higher tumour to blood ratio compared to A5B7 mF(ab')(2) used previously in the clinic, implying this new molecule may be superior for radioimmunotherapy. MIRD dose calculations showed a relatively high radiation dose to the kidney, which may limit the amount of activity that could be administered in radioimmunotherapy. However the reduction in immunogenicity was also a major advantage for A5B7 humanised divalent-Fab' maleimide over murine versions of this antibody suggesting that humanised divalent-Fab' maleimide should be a useful vehicle for repeated therapies.

Rapid and precise epitope mapping of monoclonal antibodies against Plasmodium falciparum AMA1 by combined phage display of fragments and random peptides

We describe an approach for the rapid mapping of epitopes within a malaria antigen using a combination of phage display techniques. Phage display of antigen fragments identifies the location of the epitopes, then random peptide libraries displayed on phage are employed to identify accurately amino acids involved in the epitope. Finally, phage display of mutant fragments confirms the role of each residue in the epitope. This approach was applied to the apical membrane antigen-1 (AMA1), which is a leading candidate for inclusion in a vaccine directed against the asexual blood stages of Plasmodium falciparum. As part of the effort both to understand the function of AMA1 in the parasite life cycle and to define the specificity of protective immune responses, a panel of monoclonal antibodies (MAbs) was generated to obtain binding reagents to the various domains within the molecule. There is a pressing need to determine rapidly the regions recognized by these antibodies and the structural requirements required within AMA1 for high affinity binding of the MAbs. Using phage displaying random AMA1 fragments, it was shown that MAb5G8 recognizes a short linear epitope within the pro-domain of AMA1 whereas the epitope recognized by MAb 1F9 is reduction sensitive and resides within a disulphide-bonded 57 amino acid sub-domain of domain-1. Phage displaying random peptide libraries and mutant AMA1 fragments were employed for fine mapping of the MAb5G8 core epitope to a three-residue sequence in the AMA1 prodomain.

Pathogenesis of experimental neonatal woodchuck hepatitis virus infection: chronicity as an outcome of infection is associated with a diminished acute hepatitis that is temporally deficient for the expression of interferon gamma and tumor necrosis factor-alpha messenger RNAs

Surgical biopsies of the liver were obtained from woodchuck hepatitis virus (WHV)-infected neonatal woodchucks at 2 time points before the self-limited or chronic outcomes became obvious by serologic criteria. Following segregation of outcomes, livers were analyzed for intrahepatic type 1 cytokine messenger RNAs (mRNAs) (interleukin 2 [IL-2], interferon gamma [IFN-gamma], tumor necrosis factor-alpha [TNF-alpha]) and leukocyte inflammatory phenotype (IgG+ plasma cells, lysozyme+ macrophages, CD3+ T cells). Baselines were assessed using age-matched uninfected control livers. At week 8 (early acute phase), intrahepatic type 1 cytokine mRNAs were similarly low in both outcome settings and no different from age-matched uninfected controls. This was consistent with the minimal initial viral loads and lack of histologic inflammation at this time. At week 14 (mid-acute phase), changes in viral load between outcome groups related inversely to the intrahepatic inflammatory responses. Animals that eventually became resolved had increased intrahepatic expression of IFN-gamma and TNF-alpha mRNAs and robust inflammation by CD3+ T cells, plasma cells, and macrophages. At the same time point of infection, animals that eventually became chronic carriers had an acute hepatitis involving the same cell types, but at diminished levels, and markedly deficient intrahepatic expression of IFN-gamma and TNF-alpha mRNAs. IL-2 mRNA remained at baseline control levels in both outcome groups. These cotemporal comparisons map a critical deviation in host response to the acute stage of an evolving chronic infection. They strongly suggest that increasing viral load and chronicity as an outcome of neonatal WHV infection result from a temporal deficiency in the acute intrahepatic effector mechanisms mediated by IFN-gamma and TNF-alpha.

Discovery, purification, and characterization of a temperate transducing bacteriophage for Bordetella avium

We discovered and characterized a temperate transducing bacteriophage (Ba1) for the avian respiratory pathogen Bordetella avium. Ba1 was initially identified along with one other phage (Ba2) following screening of four strains of B. avium for lysogeny. Of the two phage, only Ba1 showed the ability to transduce via an allelic replacement mechanism and was studied further. With regard to host range, Ba1 grew on six of nine clinical isolates of B. avium but failed to grow on any tested strains of Bordetella bronchiseptica, Bordetella hinzii, Bordetella pertussis, or Bordetella parapertussis. Ba1 was purified by CsCl gradient centrifugation and was found to have an icosahedral head that contained a linear genome of approximately 46.5 kb (contour length) of double-stranded DNA and a contractile, sheathed tail. Ba1 readily lysogenized our laboratory B. avium strain (197N), and the prophage state was stable for at least 25 generations in the absence of external infection. DNA hybridization studies indicated the prophage was integrated at a preferred site on both the host and phage replicons. Ba1 transduced five distinctly different insertion mutations, suggesting that transduction was generalized. Transduction frequencies ranged from approximately 2 x 10(-7) to 1 x 10(-8) transductants/PFU depending upon the marker being transduced. UV irradiation of transducing lysates markedly improved transduction frequency and reduced the number of transductants that were lysogenized during the transduction process. Ba1 may prove to be a useful genetic tool for studying B. avium virulence factors.

An outbreak of fulminant hepatitis delta in the Waorani, an indigenous people of the Amazon basin of Ecuador

An outbreak of delta hepatitis occurred during 1998 among the Waorani of the Amazon basin of Ecuador. Among 58 people identified with jaundice, 79% lived in four of 22 Waorani communities. Serum hepatitis B surface antigen (HBsAg) was found in the sera of 54% of the jaundiced persons, and 14% of asymptomatic persons. Ninety-five percent of 105 asymptomatic Waorani had hepatitis B core (HBc) IgG antibody, versus 98% of 51 with jaundice. These data confirm that hepatitis B virus (HBV) infection is highly endemic among the Waorani. Sixteen of 23 (70%) HBsAg carriers identified at the onset of the epidemic had serologic markers for hepatitis D virus (HDV) infection. All 16 were jaundiced, where as only two of seven (29%) with negative HDV serology were jaundiced (P = .0006). The delta cases clustered in families, 69% were children and most involved superinfection of people chronically infected with HBV. The data suggest that HDV spread rapidly by a horizontal mode of transmission other than by the sexual route.

Green fluorescent antibodies: novel in vitro tools

We produced a fluorescent antibody as a single recombinant protein in Escherichia coli by fusing a red-shifted mutant of green fluorescent protein (EGFP) to a single-chain antibody variable fragment (scFv) specific for hepatitis B surface antigen (HepBsAg). GFP is a cytoplasmic protein and it was not previously known whether it would fold correctly to form a fluorescent protein in the periplasmic space of E.COLI: In this study we showed that EGFP alone or fused to the N'- and C'-termini of the scFv resulted in fusion proteins that were in fact highly fluorescent in the periplasmic space of E.COLI: cells. Further characterization revealed that the periplasmic N'-terminal EGFP-scFv fusion was the most stable form which retained the fluorescent properties of EGFP and the antigen binding properties of the native scFv; thus representing a fully functional chimeric molecule. We also demonstrated the utility of EGFP-scFv in immunofluorescence studies. The results showed positive staining of COS-7 cells transfected with HepBsAg, with comparable sensitivity to a monoclonal antibody or the scFv alone, probed with conventional fluorescein-labelled second antibodies. In this study, we developed a simple technique to produce fluorescent antibodies which can potentially be applied to any scFv. We demonstrated the utility of an EGFP-scFv fusion protein for immunofluorescence studies, but there are many biological systems to which this technology may be applied.

Hepatitis delta virus: molecular biology, pathogenesis and immunology

Hepatitis delta virus (HDV) is a unique infectious agent which causes severe liver disease in those infected with its helper virus, hepatitis B virus. No effective antiviral therapy for HDV exists. This review covers recent advances in the molecular biology, pathogenesis and immunology of HDV, with an emphasis on potential targets for the development of successful antiviral agents.

Dosimetric evaluation and radioimmunotherapy of anti-tumour multivalent Fab' fragments

We have been investigating the use of cross-linked divalent (DFM) and trivalent (TFM) versions of the anti-carcinoembryonic antigen (CEA) monoclonal antibody A5B7 as possible alternatives to the parent forms (IgG and F(ab')2) which have been used previously in clinical radioimmunotherapy (RIT) studies in colorectal carcinoma. Comparative biodistribution studies of similar sized DFM and F(ab')2 and TFM and IgG, radiolabelled with both 131I and 90Y have been described previously using the human colorectal tumour LS174T nude mouse xenograft model (Casey et al (1996) Br J Cancer 74: 1397-1405). In this study quantitative estimates of radiation distribution and RIT in the xenograft model provided more insight into selecting the most suitable combination for future RIT. Radiation doses were significantly higher in all tissues when antibodies were labelled with 90Y. Major contributing organs were the kidneys, liver and spleen. The extremely high absorbed dose to the kidneys on injection of 90Y-labelled DFM and F(ab')2 as a result of accumulation of the radiometal would result in extremely high toxicity. These combinations are clearly unsuitable for RIT. Cumulative dose of 90Y-TFM to the kidney was 3 times lower than the divalent forms but still twice as high as for 90Y-IgG. TFM clears faster from the blood than IgG, producing higher tumour to blood ratios. Therefore when considering only the tumour to blood ratios of the total absorbed dose, the data suggests that TFM would be the most suitable candidate. However, when corrected for equitoxic blood levels, doses to normal tissues for TFM were approximately twice the level of IgG, producing a two-fold increase in the overall tumour to normal tissue ratio. In addition RIT revealed that for a similar level of toxicity and half the administered activity, 90Y-IgG produced a greater therapeutic response. This suggests that the most promising A5B7 antibody form with the radionuclide 90Y may be IgG. Dosimetry analysis revealed that the tumour to normal tissue ratios were greater for all 131I-labelled antibodies. This suggests that 131I may be a more suitable radionuclide for RIT, in terms of lower toxicity to normal tissues. The highest tumour to blood dose and tumour to normal tissue ratio at equitoxic blood levels was 131I-labelled DFM, suggesting that 131I-DFM may be best combination of antibody and radionuclide for A5B7. The dosimetry estimates were in agreement with RIT results in that twice the activity of 131I-DFM must be administered to produce a similar therapeutic effect as 131I-TFM. The toxicity in this therapy experiment was minimal and further experiments at higher doses are required to observe if there would be any advantage of a higher initial dose rate for 131I-DFM.

Intrafamilial transmission of hepatitis delta virus: molecular evidence

BACKGROUND/AIMS

Epidemiologic studies have suggested that transmission of hepatitis delta virus (HDV) occurs by intrafamilial routes in some populations in southern Italy, where HDV infection is endemic. To further evaluate intrafamilial transmission of HDV, we obtained the partial sequence of the viral genome from HDV-RNA positive members of families in which two or more immediate family members were positive for HDV-RNA.

METHODS

The region analyzed was the semi-conserved region from nucleotides 908 to 1265. Sequences obtained from family members were compared with those obtained from a control group of 20 unrelated patients.

RESULTS

The mean genetic divergence among HDV isolates was 2.8 +/- 1.7% within the 9 families analyzed, and 7.6 +/- 2.2% among the control group of unrelated individuals (p < 0.0001). A Receiver Operating Characteristic curve and Youden Index were used to define a cut-off value of 3.5% to discriminate sequence variations calculated within families and in the control group.

CONCLUSIONS

The data indicate that in most family units, HDV-infected members harbored nearly identical strains of HDV, and provide molecular support that HDV infection can be transmitted within the family. Such spreading among family members highlights the role of inapparent transmission through personal contacts.

A novel technique, using radioluminography, for the measurement of uniformity of radiolabelled antibody distribution in a colorectal cancer xenograft model

PURPOSE

Radioimmunotherapy of cancer employs an antitumour antibody to carry a radionuclide selectively to deposits of cancer. Conventional dose estimates, based on the Medical Internal Radiation Dose (MIRD) formulation, assume uniform distribution of radiolabelled antitumour antibody within tissue source regions. This assumption has been tested by using a statistical model to predict the pixel value distribution obtained from the digitised radioluminographs of a known radioactive source. The model uses the statistical nature of the detection of radiation where any uniform source distribution can be expected to have a detected histogram of pixel counts that is normal or Gaussian. Therefore, any test for the degree of normality in the detected distribution is also a measure of the degree of uniformity in the source.

METHODS AND MATERIALS

Three statistical techniques have been used to test the normality of the histogram of pixel values produced from the antibody distribution in a tissue section. Kurtosis, skew, and Lilliefor's are tests for normality and have statistically defined critical values for a normal distribution. After administration of 125I-labelled F(ab)2 antibody to nude mice bearing the LS174T colorectal cancer xenograft, the uniformity of antibody distribution in tumour and healthy tissues is measured using the radioluminographs of formalin-fixed paraffin sections. The test statistic for kurtosis, skew, and Lilliefor's is calculated for each tissue and is compared to critical values from statistical tables.

RESULTS

The radiolabelled antibody is distributed uniformly in liver, spleen, muscle, lung, and colon and, therefore, conforms to conventional use of the MIRD formulation. The study showed that the kidney cortex and medulla should be considered separately in macroscopic absorbed-dose calculations, as should bone marrow and hard bone. Antibody heterogeneity in the tumour necessitates the incorporation of a microdosimetric tumour model into a macrodosimetry model for the accurate calculation of absorbed dose in all tissues.

Clearance of yttrium-90-labelled anti-tumour antibodies with antibodies raised against the 12N4 DOTA macrocycle

Radioimmunotherapy (RIT) is currently limited by toxicity to normal tissues as a result of prolonged circulating radioantibody in the blood. In this study, the use of a clearing antibody was investigated (second antibody) in an attempt to reduce blood background levels of [90Y]A5B7 immunoglobulin G (IgG) activity, and, therefore, improve the therapeutic tumour-blood ratio in nude mice bearing human colorectal tumour xenografts. The second antibody was raised against the 12N4 macrocycle group used for chelation of 90Y, and is, thus, applicable to any anti-tumour antibody labelled with this methodology. Second antibody was administered 18, 24 or 48 h after radiolabelled antibody injection and produced up to a tenfold reduction in blood levels and a tenfold improvement in tumour-blood ratios. This has the advantage of reducing the risk of myelotoxicity caused by prolonged retention of activity in the blood. For all normal tissues, there was a similar or slightly lower uptake of [90Y]IgG with second antibody clearance, apart from a transient rise in liver activity due to complexes of primary and secondary antibody clearing via the liver. As a result of clearance of [90Y]IgG from the blood pool, there was an associated fall in the amount of antibody at the tumour site (up to 3.3-fold) at later time points for mice injected with second antibody. However, despite this, tumour-blood ratios remained superior to the control group at these later time points. Estimated dosimetry evaluation revealed that total dose to normal tissues, blood and tumour was lower than for the non-clearance group. Surprisingly, however, there was little improvement in total estimated tumour-blood dose ratio over the time period studied. This was probably because the majority of the dose was delivered to both the blood and tumour within the first 24 h after administration of [90Y]IgG, so that giving the clearing agent after this time did not produce a large difference in total estimated dose. The anti-macrocycle second antibody proved to be a successful clearing agent and could potentially be applied to any anti-tumour antibody coupled with the 12N4 macrocycle. In the light of the estimated dosimetry results described here, it would probably be most useful given at earlier time points (i.e. before 18 h after injection of primary antibody) to produce an improved tumour-blood ratio of total dose. Development of this strategy may allow higher levels of activity to be administered for RIT, and repeated dosing regimens.

Hepatitis D viremia following orthotopic liver transplantation involves a typical HDV virion with a hepatitis B surface antigen envelope

Patients receiving orthotopic liver transplantation (OLT) because of type D hepatitis frequently exhibit what appears to be an autonomous, or "isolated," hepatitis D virus (HDV) infection following the transplantation, with no evidence of hepatitis B virus (HBV) in the graft or in the serum. These observations have led to the hypothesis that HBV might not always be required for HDV infection, or that HDV could exist as a latent infection until rescued by HBV. Alternatively, an apparently autonomous HDV infection could be explained by coinfection of a small number of hepatocytes with both viruses following transplantation, with a very low level of HBV expression that supports low-level HDV propagation. Our results are consistent with the latter hypothesis. Sensitive polymerase chain reaction (PCR)-based analysis of HBV and HDV viremia in transplantation patients with HDV infection previously characterized as isolated showed that HDV viremia was not independent of HBV viremia. Additional analyses, including PCR amplification, buoyant density analysis in a CsCl gradient, and immunoprecipitation with monoclonal hepatitis B surface antigen antibodies (anti-HBs), indicated that the posttransplant HDV particle is typical: it contains full-length HDV RNA and an envelope of hepatitis B surface antigen (HBsAg) and is not different from that found during the acute and chronic stages of HDV superinfection or coinfection. Moreover, an experimental test of the first hypothesis in chimpanzees did not support the idea that HDV can persist for several weeks as an isolated, latent infection that can be rescued subsequently by HBV. The data indicate, therefore, that latent HDV infection is not a factor in OLT recipients. We conclude that the HDV virion in the posttransplantation setting is typical, and that HDV viremia following OLT requires the helper function of HBV infection.

Hepatitis delta virus RNA editing is highly specific for the amber/W site and is suppressed by hepatitis delta antigen

RNA editing at adenosine 1012 (amber/W site) in the antigenomic RNA of hepatitis delta virus (HDV) allows two essential forms of the viral protein, hepatitis delta antigen (HDAg), to be synthesized from a single open reading frame. Editing at the amber/W site is thought to be catalyzed by one of the cellular enzymes known as adenosine deaminases that act on RNA (ADARs). In vitro, the enzymes ADAR1 and ADAR2 deaminate adenosines within many different sequences of base-paired RNA. Since promiscuous deamination could compromise the viability of HDV, we wondered if additional deamination events occurred within the highly base paired HDV RNA. By sequencing cDNAs derived from HDV RNA from transfected Huh-7 cells, we determined that the RNA was not extensively modified at other adenosines. Approximately 0.16 to 0.32 adenosines were modified per antigenome during 6 to 13 days posttransfection. Interestingly, all observed non-amber/W adenosine modifications, which occurred mostly at positions that are highly conserved among naturally occurring HDV isolates, were found in RNAs that were also modified at the amber/W site. Such coordinate modification likely limits potential deleterious effects of promiscuous editing. Neither viral replication nor HDAg was required for the highly specific editing observed in cells. However, HDAg was found to suppress editing at the amber/W site when expressed at levels similar to those found during HDV replication. These data suggest HDAg may regulate amber/W site editing during virus replication.

Genotype-specific complementation of hepatitis delta virus RNA replication by hepatitis delta antigen

Characterizations of genetic variations among hepatitis delta virus (HDV) isolates have focused principally on phylogenetic analysis of sequences, which vary by 30 to 40% among three genotypes and about 10 to 15% among isolates of the same genotype. The significance of the sequence differences has been unclear but could be responsible for pathogenic variations associated with the different genotypes. Studies of the mechanisms of HDV replication have been limited to cDNA clones from HDV genotype I, which is the most common. To perform a comparative analysis of HDV RNA replication in genotypes I and III, we have obtained a full-length cDNA clone from an HDV genotype III isolate. In transfected Huh-7 cells, the functional roles of the two forms of the viral protein, hepatitis delta antigen (HDAg), in HDV RNA replication are similar for both genotypes I and III; the short form is required for RNA replication, while the long form inhibits replication. For both genotypes, HDAg was able to support replication of RNAs of the same genotype that were mutated so as to be defective for HDAg production. Surprisingly, however, neither genotype I nor genotype III HDAg was able to support replication of such mutated RNAs of the other genotype. The inability of genotype III HDAg to support replication of genotype I RNA could have been due to a weak interaction between the RNA and HDAg. The clear genotype-specific activity of HDAg in supporting HDV RNA replication confirms the original categorization of HDV sequences in three genotypes and further suggests that these should be referred to as types (i.e., HDV-I and HDV-III) rather than genotypes.

Cloning and characterization of partial cDNAs for woodchuck cytokines and CD3epsilon with applications for the detection of RNA expression in tissues by RT-PCR assay

Immunologic reagents and methodology are essential to develop further the woodchuck and woodchuck hepatitis virus (WHV) as a model of immune response, inflammation, and immunotherapy in hepatitis B virus (HBV) infection. Partial cDNA clones for the woodchuck CD3epsilon marker of T cells (536 bp) and for selected woodchuck cytokines were developed, including IL-1beta (332 bp), IL-2 (249 bp), IL-4 (205 bp), IL-10 (476 bp), IFN-gamma (476 bp), and TNF-alpha (381 bp). This panel of markers includes sets to measure RNAs for T cells (CD3epsilon), immune response induction (IL-1beta, IL-2), TH subsets (TH1, IL-2/IFN-gamma vs. TH2, IL-4/IL-10), and effector molecules that regulate hepadnavirus replication and liver injury (IFN-gamma, TNF-alpha). Primers representing highly conserved segments of genes from other species were used to derive the partial cDNA clones. Target RNA was obtained from woodchuck peripheral blood mononuclear cells (PBMC) that were stimulated in vitro with ConA, LPS, and human rIL-2. The cDNA clones were validated by 1) comparison with other species for homologies in the nucleotide and predicted amino acid sequences and 2) a first generation assay demonstrating induction of the respective RT-PCR products in stimulated woodchuck PBMC. The corresponding RNAs were also detectable in most cases in the total RNA from the livers of uninfected and WHV-infected woodchucks and differential expression of IFN-gamma and TNF-alpha RNAs was suggested. Second generation, semi-quantitative assays for the RNAs were validated using RT-PCR and dot-blot hybridization with 32P-oligomers derived from the internal sequences of the respective clones. Continued study of the woodchuck immune response to WHV infection using these assays will provide insight into the kinetics and immune mechanisms that initiate and maintain chronic hepadnavirus infection and, hence, enable development of improved immunotherapies for established chronic HBV infection.

Cloning and characterization of partial cDNAs for woodchuck cytokines and CD3epsilon with applications for the detection of RNA expression in tissues by RT-PCR assay

Immunologic reagents and methodology are essential to develop further the woodchuck and woodchuck hepatitis virus (WHV) as a model of immune response, inflammation, and immunotherapy in hepatitis B virus (HBV) infection. Partial cDNA clones for the woodchuck CD3epsilon marker of T cells (536 bp) and for selected woodchuck cytokines were developed, including IL-1beta (332 bp), IL-2 (249 bp), IL-4 (205 bp), IL-10 (476 bp), IFN-gamma (476 bp), and TNF-alpha (381 bp). This panel of markers includes sets to measure RNAs for T cells (CD3epsilon), immune response induction (IL-1beta, IL-2), TH subsets (TH1, IL-2/IFN-gamma vs. TH2, IL-4/IL-10), and effector molecules that regulate hepadnavirus replication and liver injury (IFN-gamma, TNF-alpha). Primers representing highly conserved segments of genes from other species were used to derive the partial cDNA clones. Target RNA was obtained from woodchuck peripheral blood mononuclear cells (PBMC) that were stimulated in vitro with ConA, LPS, and human rIL-2. The cDNA clones were validated by 1) comparison with other species for homologies in the nucleotide and predicted amino acid sequences and 2) a first generation assay demonstrating induction of the respective RT-PCR products in stimulated woodchuck PBMC. The corresponding RNAs were also detectable in most cases in the total RNA from the livers of uninfected and WHV-infected woodchucks and differential expression of IFN-gamma and TNF-alpha RNAs was suggested. Second generation, semi-quantitative assays for the RNAs were validated using RT-PCR and dot-blot hybridization with 32P-oligomers derived from the internal sequences of the respective clones. Continued study of the woodchuck immune response to WHV infection using these assays will provide insight into the kinetics and immune mechanisms that initiate and maintain chronic hepadnavirus infection and, hence, enable development of improved immunotherapies for established chronic HBV infection.

Geographic distribution and genetic variability of hepatitis delta virus genotype I

Three genotypes of hepatitis delta virus (HDV) have been identified, each with different geographic distributions and disease associations. To better define the geographic distribution and genetic variability of HDV genotype I, and to evaluate the extent of genome variability in populations with different patterns of HDV infection, we have analyzed the sequence of HDV RNA in the sera of 72 patients from different areas. Patients were primarily residents of the United States and areas in and around Greece, including Archangelos, Rhodes. All sequences obtained belonged to HDV genotype I, confirming the wide geographic distribution of this genotype and its predominance in Europe and the United States. In contrast to previous studies, phylogenetic analysis of this large and diverse group of sequences, along with all available previously published HDV sequences, showed no well-defined subtypes within genotype I. Low sequence diversity was found for isolates from the United States, Archangelos, Turkey, and Albania, suggesting that HDV was introduced more recently and/or from fewer sources into these areas as compared to mainland Greece, Italy, and north Africa, where sequence diversity is much greater. The low sequence diversity among isolates from Archangelos is particularly interesting in light of the unusually mild pattern of HDV disease found in this community. Comparison of nucleic acid and amino acid sequences within and among genotypes indicated both highly conserved regions as well as genotype-specific sequences that could be related to functional differences. The most distinctive of the latter was that corresponding to the C-terminal 19-20 amino acids of the long form of hepatitis delta antigen, which is highly conserved within each genotype but considerably diverged among them.

The predominance of hepatitis delta virus genotype I among chronically infected Italian patients

The sera of 46 Italian patients with chronic hepatitis delta virus (HDV) infection were analyzed for HDV RNA by polymerase chain reaction (PCR) amplification. Genetic analysis of sequences amplified from two regions of the HDV genome indicated that all HDV RNA-positive patients (98%) were infected with HDV genotype I. In Italy, infection with this genotype appeared to be associated with a broad spectrum of chronic disease. No subtypes of HDV genotype I were identified, nor were genetic variations clearly associated with different disease patterns; however, clustering of some sequences suggested correlations with geography and transmission route. Italian HDV genotype I sequences were more diverse than those from east Asia and North America, suggesting that HDV genotype I was likely introduced to Italy earlier and/or from multiple sources as compared to those areas. All sequences analyzed were predicted to fold into the unbranched rod structure typical of HDV RNA. Within this structure, three conserved features were identified, including sequences around the RNA editing site and the polyadenylation signal site. We conclude that in Italy, where HDV infection has been endemic, the overwhelmingly predominant genotype of HDV is genotype I.

Hepatitis B virus (HBV)/hepatitis D virus (HDV) coinfection in outbreaks of acute hepatitis in the Peruvian Amazon basin: the roles of HDV genotype III and HBV genotype F

Recurring outbreaks of acute hepatitis have been a significant cause of morbidity and mortality among Peruvian military personnel stationed in the Amazon Basin region of Peru. The role of hepatitis B virus (HBV) and hepatitis D virus (HDV) infection was investigated as the possible cause of acute hepatitis among 88 military patients stationed at four different jungle outposts during 1992-1993. Analysis of serum markers indicated that 95% (84/88) had evidence of acute HBV infection; 64% (54/84) were also infected with HDV. Genetic analysis of PCR-amplified HDV and HBV fragments showed exclusively HDV genotype III and HBV genotype F. Furthermore, HDV RNA sequences were similar among patients from the same outpost but different from those at other jungle locations. The data suggested focal sources of HDV infection in the jungle environment of the outposts and, further, confirmed the unique association of HDV genotype III with severe cases of human disease in northern South America.

Preparation, characterisation and tumour targeting of cross-linked divalent and trivalent anti-tumour Fab' fragments

The monoclonal anti-CEA antibody, A5B7, has previously been administered to patients for radioimmunotherapy (RIT). Long circulation time and the formation of an immune response have limited therapeutic success in the clinic. Antibody fragments can be used to reduce the in vivo circulation time, but the best combination of fragment and radioisotope to use for therapy is far from clear. In this study we have compared the biodistribution of A5B7 IgG and F(ab')2 with chemically cross-linked divalent (DFM) and trivalent (TFM) A5B7 Fab' fragments in nude mice bearing human colorectal tumour xenografts. The cross-linkers were designed to allow site-specific labelling using yttrium 90 (90Y), a high-energy beta-emitter. We have also compared the above antibody forms conjugated to both 131I and 90Y. Both DFM and TFM were fully immunoreactive and remained intact after radiolabelling and incubation in serum at 37 degrees C for 24 h. Biodistribution results showed similar tumour uptake levels and an identical blood clearance pattern for F(ab')2 and DFM with high tumour-blood ratios generated in each case. However, unacceptably high kidney accumulation for both F(ab')2 and DFM and elevated splenic uptake of DFM labelled with 90Y was observed. Kinetic analysis of antigen binding revealed that DFM had the fastest association rate (kass = 1.6 x 10(5) Ms-1) of the antibody forms, perhaps owing to increased flexibility of the cross-linker. This advantage implies that DFM may be more suitable than F(ab')2 radiolabelled with 131I for RIT. TFM cleared from the blood significantly faster than A5B7 IgG when labelled with both 131I and 90Y, producing an improved therapeutic tumour-blood ratio. Kidney accumulation was not observed for [90Y]TFM, but a slightly higher splenic uptake was observed that may indicate reticuloendothelial system (RES) uptake. Overall, tumour uptake was higher for 90Y-labelled antibodies than for 131I-labelled antibodies. Because of the faster clearance, it should be possible to administer a higher total dose of 90Y-labelled TFM than IgG, which is attractive for RIT. Both A5B7 DFM and TFM, therefore, show favourable properties compared with their parent antibody forms.

Clinical evidence of efficient tumor targeting based on single-chain Fv antibody selected from a combinatorial library

We present a system for cancer targeting based on single-chain Fv (scFv) antibodies selected from combinatorial libraries, produced in bacteria and purified by using an engineered tag. Combinatorial libraries of scFv genes contain great diversity, and scFv antibodies with characteristics optimized for a particular task can be selected from them using filamentous bacteriophage. We illustrate the benefits of this system by imaging patients with carcinoembryonic antigen (CEA)-producing cancers using an iodine-123 labeled scFv anti-CEA selected for high affinity. All known tumor deposits were located, and advantages over current imaging technology are illustrated. ScFvs are produced in a cloned form and can be readily engineered to have localizing and therapeutic functions that will be applicable in cancer and other diseases.

Hepatitis delta virus. Genetics and pathogenesis

After the discovery of HDV there have been significant advances in the understanding of the biology and disease of HDV infection. Analyses at the molecular level have revealed several fascinating features (ribozyme activity, RNA-dependent RNA polymerase activity of RNA polymerase II, HDAg isoprenylation, and RNA editing) that are of significant interest. Intensive investigation of the ribozyme elements has yielded important insights in both functional and structural features. However, there is information lacking about other aspects of the HDV replication cycle including the specific nature of the interaction between HDAg and HDV RNA, the function of HDAg in HDV RNA replication, transcription by RNA polymerase II, and the mechanisms of HDV RNA editing and its regulation. Further study of these and other aspects of the HDV replication cycle will continue to enrich our understanding of basic biology. Evaluation of the mechanisms of HDV disease remains an important goal in the study of this agent. Although both acute and chronic disease are commonly associated with unfavorable outcomes, it is clear that chronic infection is associated with a broad spectrum of disease. The interactions between HDV, HBV, and the host are necessarily complex, and it is likely that each contribute factors that influence disease and outcome. Recent analyses of HDV genotypes have suggested that disease variations may be associated with viral genetic factors. Consistent with the obligate role of HBV in the HDV life cycle, HBV replication is also an important determinant of HDV disease. It is still unclear if interactions between specific genotypes or variants of HBV and HDV influence disease. Recent data also suggest that infection with multiple hepatitis viruses (HBV, HDV, and HCV) can influence the severity of disease. It remains to be seen whether coinfection with the recently discovered hepatitis G virus is associated with altered disease patterns. Further advances in our understanding HDV disease and possible therapeutic approaches will rely on a combination of additional studies at the molecular, genetic, epidemiologic, and clinical levels. These studies will continue to elaborate the model of HDV infection and disease that can ultimately be tested by experimental infection of chimpanzees and woodchucks.

Technetium-99m radiolabeling using a phage-derived single-chain Fv with a C-terminal cysteine

Single-chain Fv (scFv) antibody fragments have potential for clinical imaging because of their rapid tumor penetration and high tumor-to-tissue ratios at early time points. ScFvs clear rapidly from the circulation so radiolabels such as 99mTc which have short half-lives are desirable, but the free thiol groups necessary for labeling with 99mTc are not normally found on these molecules.

METHODS

We constructed a vector which enabled a free cysteine to be linked to the C-terminus of scFvs. MFE-23, a scFv directed against carcinoembryonic antigen (CEA), was cloned into this vector and cys-tagged MFE-23 was labeled with 99mTc using a D-glucarate transfer method.

RESULTS

The radiolabeled product was stable in vivo and in vitro and showed favorable tumor-to-blood ratios in vivo at early time points (4:1 at 24 hr and 8:1 at 48 hr), although high kidney levels were also detected.

CONCLUSION

Our study demonstrates an effective method to enable scFvs radiolabeling with 99mTc and also shows the potential of using a 99mTc-labeled scFv for clinical imaging studies.

RNA editing of hepatitis delta virus antigenome by dsRNA-adenosine deaminase

Hepatitis delta virus (HDV) is a subviral human pathogen that requires hepatitis B virus (HBV) for packaging. Concurrent infection by HBV and HDV increases the risk of severe liver disease compared to infection with HBV alone. The HDV genome is a closed circular RNA of about 1,700 bases which is replicated through an RNA intermediate, the antigenome. Both RNAs can be folded into highly base-paired, rod-shaped structures, similar to the plant viroid RNAs. Two forms of the sole HDV protein, hepatitis delta antigen, are derived from a single open reading frame by RNA editing; the enzymes responsible for the editing have not been characterized. Here we report that the purified enzyme dsRAD (for double-stranded-RNA-adenosine deaminase) can edit HDV antigenomic RNA in vitro. Most important, we observe that mutations in critical sequences of the antigenome have identical effects on in vitro and in vivo editing, suggesting that dsRAD, or a closely related enzyme, is responsible for editing HDV RNA in vivo.

Hepatitis D virus RNA editing: specific modification of adenosine in the antigenomic RNA

RNA editing plays a central role in the life cycle of hepatitis D virus (HDV), a subviral human pathogen. Previous studies (J.L. Casey, K.F. Bergmann, T.L. Brown, and J.L. Gerin, Proc. Natl. Acad. Sci USA 89:7149-7153, 1992; H. Zheng, T.-B. Fu, D. Lazinski, and J. Taylor, J. Virol. 66:4693-4697, 1992) had concluded that the genomic RNA of HDV was the target for RNA editing and that the editing reaction was a conversion of U to C. However, we show here that the antigenomic RNA of HDV is in fact the target for HDV RNA editing, which is therefore a conversion of A to G. This result is verified by using an assay specific for editing on the antigenomic RNA and by analyzing the editing of site-directed mutant RNAs in transfected cells and in cell extracts. Because editing occurs in the absence of viral antigens and the specificity for the HDV editing target site is present even in extracts from Drosophila cells, it is likely that HDV RNA is edited by one or more cellular factors that are conserved among higher eukaryotes. These results raise the likelihood that double-stranded RNA adenosine deaminase specifically edits HDV antigenomic RNA.

Purification of bacterially expressed single chain Fv antibodies for clinical applications using metal chelate chromatography

A new procedure is described for the purification of an anti-carcinoembryonic antigen (CEA) single chain Fv (scFv), referred to as MFE-23, from bacterial supernatant. A simple insertion of a hexa-histidine tail fused at the C-terminus (MFE-23 His) provides an affinity tag which selectively binds to transition metal ions immobilised on an iminodiacetic acid (IDA) derivitised solid phase matrix. This method proved to be superior to standard CEA antigen affinity chromatography in the following ways. (1) A higher yield was produced (10 mg/l as opposed to 2.2 mg/l of bacterial supernatant). The latter figure was largely affected by the limited availability (size of the column) of immobilised CEA antigen. (2) Scale-up was relatively simple and less costly. (3) The risk of tumour derived antigen leaching from the column is eliminated. Results showed that immobilised Cu2+ ions were more effective than Ni2+ and Zn2+ ions in retaining the His tagged product giving a 90% pure product on elution. Clinical grade material was generated using size exclusion chromatography to remove aggregated material, and Detoxi gel to remove bacterial endotoxins. Validation assays to measure DNA, copper and endotoxins were performed to assess the levels of contaminants. MFE-23 His retained 84% antigen binding after 6 months storage at 4 degrees C and > 75% after radiolabelling. Further experiments confirmed that the His tail did not affect biodistribution and tumour localisation in nude mice bearing human colorectal tumour xenografts.

Radioimmunotherapy of metastatic colorectal tumours with iodine-131-labelled antibody to carcinoembryonic antigen: phase I/II study with comparative biodistribution of intact and F(ab')2 antibodies

Studies in animal tumour models of colorectal cancer suggest that F(ab')2 antibody fragments to carcinoembryonic antigen (CEA) labelled with iodine-131 give superior therapy compared with intact anti-CEA antibody. The purpose of this study was to investigate this hypothesis in patients. Ten patients received intact A5B7 IgG1 mouse monoclonal antibody (MAb) to CEA and nine patients received the F(ab')2 fragment of the same antibody. The biodistribution for each molecule was compared using quantitative single-photon emission computerised tomographic (SPECT) gamma-camera imaging. Tumour responses were seen in both groups and myelosuppression was the limiting toxicity. F(ab')2 localised more rapidly than intact antibody in tumour, giving a mean percentage injected activity per kg at 4.25 h after injection of 8.2% for F(ab')2 compared with 4.4% for intact antibody (P < 0.05). No significant difference in antibody clearance from, or cumulative dose per unit administered activity (cGy MBq-1) to, tumour was seen. Distribution in blood was similar for both the intact and fragment antibody. These findings are consistent with more rapid penetration of the smaller F(ab')2 into tumour masses. More efficient early uptake will give higher maximum dose rates to the tumour which is valuable for radioimmunotherapy (RIT) when low dose rates may limit effectiveness of treatment. F(ab')2 fragments may provide a substantially enhanced method of delivering RIT.

A genotype of hepatitis D virus that occurs in northern South America

Hepatitis D virus (HDV) is the cause of an unusually severe form of liver disease with distinct histologic features (morula cell) that occurs throughout northern South America and certain other areas of the world. Clinical studies of HDV disease worldwide indicate that there is, in fact, a wide variation in pathogenesis, and the reasons for these differences are presently unknown. One possible explanation is that factors associated with the viral genotype are determinants of HDV pathogenesis. In this study, nucleic acid sequences were determined for three different northern South American HDV isolates which were obtained from individuals with severe disease or a family history of severe disease, in areas that are hyperendemic for this disease pattern. The sequences of these three isolates are very similar to one another but only distantly related to other published HDV sequences. Comparison of the sequence of a semiconserved region from a total of 14 isolates indicates that there are at least three HDV genotypes. Most published HDV sequences, including those from North America, Europe, the Middle East, the South Pacific, and Asia, belong to a single genotype which may have some geographically based subtypes. A single Japanese isolate is the sole representative of a second HDV genotype. The South American sequences reported here constitute a third genotype. The association of a particular genotype with the severe form of type D hepatitis that occurs in northern South America supports the hypothesis that HDV genetic factors are important determinants in the pathogenesis of type D hepatitis.

Structural requirements for RNA editing in hepatitis delta virus: evidence for a uridine-to-cytidine editing mechanism

Hepatitis delta virus (HDV) nucleotide 1012 is edited from uridine to cytidine in 10-40% of the RNA genomes during replication. This editing event is an important control point in the HDV life cycle because it results in both the packaging of viral RNA and the inhibition of HDV replication. We find that the editing event is highly specific for both the sequences neighboring nucleotide 1012 and the base-paired context of position 1012 within the unbranched rod structure of HDV RNA. Prior studies identified the base transition at nucleotide 1012 but were unable to distinguish between editing of the genomic versus the antigenomic strands [Luo, G. X., Chao, M., Hsieh, S. Y., Sureau, C., Nishikura, K. & Taylor, J. (1990) J. Virol. 64, 1021-1027]. In this study, comparisons of mutations that differentiate between base pairing in genomic and antigenomic RNAs indicate that the genomic strand of HDV is the actual editing substrate. We conclude that the virus uses a uridine to cytidine editing mechanism, which is provided by the host cell.

Autoregulation of human thymidylate synthase messenger RNA translation by thymidylate synthase

Thymidylate synthase (TS; 5,10-methylenetetrahydrofolate:dUMP C-methyltransferase, EC 2.1.1.45) is essential for the de novo synthesis of thymidylate, a precursor of DNA. Previous studies have shown that the cellular level of this protein is regulated at both the transcriptional and posttranscriptional levels. The regulation of human TS mRNA translation was studied in vitro with a rabbit reticulocyte lysate system. The addition of purified human recombinant TS protein to in vitro translation reactions inhibited translation of TS mRNA. This inhibition was specific in that recombinant TS protein had no effect on the in vitro translation of mRNA for human chromogranin A, human folate receptor, preplacental lactogen, or total yeast RNA. The inclusion of dUMP, 5-fluoro-dUMP, or 5,10-methylene-tetrahydrofolate in in vitro translation reactions completely relieved the inhibition of TS mRNA translation by TS protein. Gel retardation assays confirmed a specific interaction between TS protein and its corresponding mRNA but not with unrelated mRNAs, including human placenta, human beta-actin, and yeast tRNA. These studies suggest that translation of TS mRNA is controlled by its own protein end product, TS, in an autoregulatory manner.

Translation and the stability of mRNAs encoding the transferrin receptor and c-fos

Turnover of the full-length human transferrin receptor (TfR) mRNA is regulated by iron, and this regulation is mediated by the transcript's 3' untranslated region. Alterations in the sequence of the TfR mRNA regulatory region have been identified that render the mRNA unregulated by iron and intrinsically unstable. When cells expressing this unstable mRNA are treated with inhibitors of protein synthesis (cycloheximide or puromycin), the steady-state level of the encoded human TfR mRNA is increased due to a stabilization of the transcript. A similar set of observations has been made using a chimeric mRNA in which the rapid turnover determinant of the TfR mRNA is replaced by the (A+U)-rich region from the 3' untranslated region of c-fos mRNA. To distinguish between a labile protein participant in the degradation of these mRNAs and a requirement for their translation per se, we introduced a ferritin iron-responsive element into the 5' untranslated region of each of these mRNAs. The presence of the 5' iron-responsive element allowed us to use iron availability to alter the translation of the mRNAs in question without global effects on cellular protein synthesis. Although specific translation of these mRNAs could be inhibited by iron chelation to a degree comparable to that seen with cycloheximide (approximately 95% inhibition), no effects on mRNA turnover were observed. These data support a model in which a trans-acting labile protein is necessary for the turnover of these mRNAs rather than there being a requirement for the translation of the mRNAs themselves.

Iron regulation of transferrin receptor mRNA levels requires iron-responsive elements and a rapid turnover determinant in the 3′ untranslated region of the mRNA

Post-transcriptional regulation of transferrin receptor mRNA levels by iron is mediated by a portion of the 3' untranslated region (UTR) of the mRNA. We have previously shown that a 678 nucleotide fragment of the 3'UTR contains the regulatory element(s). Within this region are five RNA structures which resemble the iron-responsive element (IRE) in the 5' untranslated region of the ferritin mRNA which is regulated translationally by iron. The IREs from the ferritin and transferrin receptor mRNAs compete in an in vitro assay for interaction with a cytoplasmic protein; the activity of this IRE-binding protein is dependent upon the iron status of the cells. Based on further deletion analysis reported here, the sequence required for iron regulation of the transferrin receptor have been limited to 250 nucleotides which we have produced synthetically and cloned. This sequence, which contains three IREs, is capable of producing iron-dependent regulation of transferrin receptor levels. Removal of the three IREs from the synthetic element results in loss of iron regulation. Moreover, deletion of a single cytosine residue from each of the three IREs in the synthetic regulatory element eliminates high-affinity binding to the IRE-binding protein in vitro and results in low levels of iron-independent transferrin receptor expression, consistent with production of a constitutively unstable mRNA. These data indicate that the ability of the mRNA to interact with the IRE-binding protein is required for regulation of transferrin receptor mRNA levels by iron.(ABSTRACT TRUNCATED AT 250 WORDS)

A cytosolic protein binds to structural elements within the iron regulatory region of the transferrin receptor mRNA

The level of mRNA encoding the transferrin receptor (TfR) is regulated by iron, and this regulation is mediated by a portion of the 3' untranslated region (UTR) of the TfR transcript. This portion of 3' UTR of the human TfR mRNA contains five RNA elements that have structural similarity to the iron-responsive element (IRE) found as a single copy in the 5' UTR of the mRNA for ferritin, whose translation is regulated by iron. Moreover, five very similar elements are also contained in the 3' UTR of the chicken TfR mRNA. Cytosolic extracts of human cell lines are shown by a gel shift assay involving RNase T1 protection to contain an IRE-binding protein capable of specific interaction with the human TfR 3' UTR. When the protecting protein is removed, the protected RNA can be digested with RNase T1 to yield oligoribonucleotide fragments characteristic of two of the IREs contained in the TfR 3' UTR. As judged by cross-competition experiments, the same IRE-binding protein interacts with the ferritin IRE. The apparent affinity of RNA sequence elements for the IRE-binding protein is shown to depend upon the sequence of the RNA. A comprehensive secondary structure for the regulatory region of the TfR mRNA is proposed based on the experimentally demonstrated presence of at least two IRE-like structural elements.

A model for the structure and functions of iron-responsive elements

Most eukaryotic cells express two proteins, whose biosynthetic rates are determined by the intracellular iron status. The genes for both these proteins, ferritin and the transferrin receptor (TfR), are regulated at the post-transcriptional level, but by entirely different mechanisms. Ferritin mRNA levels are not affected by acute changes in iron availability. Ferritin biosynthesis is regulated translationally via a defined element contained within the 5' untranslated region (UTR) of the ferritin mRNA. This element has been highly conserved during evolution and has been termed an iron-responsive element (IRE). In contrast to ferritin, the regulation of TfR biosynthesis is mirrored by equivalent changes in TfR mRNA levels. The genetic information for this regulation is mostly located in the region of the gene encoding the 3' UTR of the TfR mRNA. Five elements that closely resemble the ferritin IRE are contained within the region which is critical for TfR regulation. The IRE is suggested to function by forming a specific stem-loop structure that interacts with a transacting factor in an iron-dependent fashion. We present a model that accommodates the mediation of distinct post-transcriptional regulatory phenomena via IREs.

The promoter region of the human transferrin receptor gene

Genomic DNA fragments corresponding to the promoter region of the human transferrin receptor were linked to either the full-length receptor cDNA or to the bacterial enzyme chloramphenicol acetyltransferase. These constructs were transfected into mouse and human cells, respectively. Gene expression was monitored 40-48 hours after transfection. Bal31 exonuclease was employed to produce 5' to 3' deletions of the promoter region. Deletion of DNA between -86 and -70 upstream of the receptor's mRNA start site resulted in a greater than 80% reduction in apparent promoter activity. DNA sequencing of the 150 bp upstream of the start site revealed that the promoter region contained several sequence elements more than 90% homologous to the consensus sequence for binding of the transcription factor Sp1. In addition, an 11 bp sequence identical to a segment of the enhancers of polyoma virus and adenovirus was located between -80 and -70. Internal deletions confirmed that this enhancer homologue was critical for full promoter activity. A 66 bp fragment encompassing the -80/-70 element augmented gene expression when the fragment was placed in either orientation upstream of the remainder of the transferrin receptor promoter.

Iron-responsive elements: regulatory RNA sequences that control mRNA levels and translation

The biosynthetic rates for both the transferrin receptor (TfR) and ferritin are regulated by iron. An iron-responsive element (IRE) in the 5' untranslated portion of the ferritin messenger RNA (mRNA) mediates iron-dependent control of its translation. In this report the 3' untranslated region of the mRNA for the human TfR was shown to be necessary and sufficient for iron-dependent control of mRNA levels. Deletion studies identified a 678-nucleotide fragment of the TfR complementary DNA that is critical for this iron regulation. Five potential stem-loops that resemble the ferritin IRE are contained within the region critical for TfR regulation. Each of two of the five TfR elements was independently inserted into the 5' untranslated region of an indicator gene transcript. In this location they conferred iron regulation of translation. Thus, an mRNA element has been implicated in the mediation of distinct regulatory phenomena dependent on the context of the element within the transcript.

Two genetic loci participate in the regulation by iron of the gene for the human transferrin receptor

Iron regulation of the human transferrin receptor gene was examined in murine cells transformed with chimeric constructs containing the human transferrin receptor gene's promoter and either the structural gene for bacterial chloramphenicol acetyltransferase or the human transferrin receptor cDNA. The activity of the transferrin receptor gene's promoter with the heterologous indicator gene was found to be approximately equal to 3-fold higher in cells treated with the iron chelator desferrioxamine than in cells treated with the iron source, hemin. A higher degree of iron regulation was seen in the expression of the human transferrin receptor cDNA driven by its own promoter. The receptor cDNA under the control of the simian virus 40 early promoter was also iron-regulated. Several human transferrin receptor transcripts differing in their 3' end were produced in the murine cells regardless of the promoter used, with the shorter transcripts being relatively unregulated by iron. Deletion of cDNA corresponding to most of the 3' untranslated portion of the mRNA for the receptor ablated the iron regulation. We conclude that at least two genetic elements exist for the regulation of the transferrin receptor gene by iron. One has its locus in the DNA upstream of the transferrin receptor gene's transcription start site, and the other is dependent upon the integrity of the sequences in the 3' end of the gene.

Deletional analysis of the promoter region of the human transferrin receptor gene

Fragments of human genomic DNA corresponding to the promoter region of the gene for the transferrin receptor have been cloned upstream of the bacterial gene for chloramphenicol acetyltransferase and these constructs used to assess promoter activity following transfection into a human rhabdomyosarcoma cell line. Progressive 5' deletions as well as internal linker-substitution constructs support a critical role in gene expression of a sequence element approximately 70 bp upstream of the mRNA start site. In this region, the receptor gene was found to contain 11bp that are identical to a segment of the enhancers of polyoma virus and adenovirus. A fragment encompassing this element was shown to increase gene expression when the fragment was placed in either orientation upstream of the remainder of the transferrin receptor promoter but the same fragment did not activate an enhancer-less SV40 promoter. Removal from within the receptor promoter of three potential binding sites for the transcription factor Sp1 did not decrease the promoter's activity.

Thermodynamics of the binding of D-glucose to yeast hexokinase

The binding of D-glucose to baker's yeast hexokinase (EC 2.7.1.1, ATP:D-hexose 6-phosphotransferase) was studied by isothermal and differential scanning calorimetry (DSC) and by fluorometric titration. The enthalpy and heat capacity changes associated with the binding of glucose were found to be nearly zero at both low and high ionic strengths over the temperature range from 7 to 29 degrees C. Thus, the free-energy change, amounting to -5.1 kcal mol(-1) at 25 degrees C and high ionic strength, is nearly independent of the temperature and is primarily of entropic origin. DSC study of the thermal unfolding of the free enzyme at low ionic strength gave an excess heat capacity curve with two maxima. This result appears to reflect a difference in thermal stability of the two domains in the hexokinase molecule which are indicated by X-ray crystallography [Bennett, W.S., & Steitz, T. A. (1978) Proc. Natl. Acad. Sci. U.S.A. 75, 4848-4852]. In contrast, the unfolding of free enzyme at high ionic strength was fully cooperative. The excess heat capacity curve for the unfolding of the glucose-bound enzyme had only one peak at both low and high ionic strengths. This is consistent with the X-ray result that the binding of glucose induces a conformational change in the enzyme which brings the two lobes into close proximity. It is interesting that such a significant, molecule-wide conformational change is accompanied by only very small net changes in enthalpy and heat capacity.